1. Field of the Invention
The present invention relates to a photodiode array, a method of making the same, and a radiation detector.
2. Related Background Art
Packaging a photodiode array for CT is needed to be done three-dimensionally. Three-dimensional packaging requires signals to be outputted from the opposite side from the light-incident side, for which a back-illuminated photodiode array is used in general.
When the distance between a pn junction and the light-incident surface is long in a back-illuminated photodiode array, carriers generated within a substrate are recombined in the process of migrating to the pn junction and thus cannot be taken out as a signal. As a consequence, for improving the detection sensitivity, it is necessary that the distance between the pn junction and the light-incident surface be as short as possible.
A back-illuminated photodiode array reducing this distance has been proposed (Japanese Patent Application Laid-Open No. HEI 7-333348).
FIG. 15 is a side sectional view of this photodiode array.
In this photo diode array 101, a p-type diffusion layer 105 is formed like a rectangular column extending from one side of a substrate to an n-type layer 103.
Since the p-type diffusion layer 105 is formed by injecting impurities, however, it is difficult for such an impurity layer to be formed uniformly by a thickness yielding a sufficient sensitivity.
Thus, the above-mentioned photodiode array is disadvantageous in that it is hard to manufacture.
When the photodiode array as a whole is formed thinner, its mechanical strength is hard to keep, whereby the photodiode array is likely to break in subsequent steps.
Therefore, a technique for partly thinning the photodiode array may be considered. Namely, only regions formed with photodiodes may be thinned, so as to reduce the distance between the pn junction and the light-incident surface while keeping the mechanical strength.
FIG. 16 is a side sectional view of this photodiode array.
In this photodiode array, only regions formed with a p-type diffusion layer 105 in an n-type layer 103 are thinned from the side on which light to be detected is incident, whereas the remaining regions are left with the original thickness of the semiconductor substrate as a frame, so as to maintain the mechanical strength. In this photodiode array, depressions of the n-type substrate are formed at positions corresponding to respective pn junctions from the side (backside) opposite from the side (front side) formed with the pn junctions. Namely, one depression is formed for each pn junction pixel. A projection is formed between a pn junction pixel and its adjacent pn junction pixel.